The bcl-2 gene encodes a prototypic intracellular survival factor that inhibits apoptotic cell death and is tightly regulated in vivo. A family of bcl-2-related genes has recently been discovered that are either positive or negative regulators of cell death with at least five members of this family expressed during hematopoiesis. We have recently ascertained that, although bcl-2 expression is present in most hematopoietic precursors, it is absent in a primitive subset of CD34+ CD38 - lin- precursors of small to medium lymphocyte size. We now propose to characterize the roles of bcl-2 and bcl-2-related genes in early hematopoietic development through four related specific aims. First, we will map the expression of the different bcl-2 -related genes in primary human hematopoietic cells. Second, we will try to determine whether an alternative survival mechanism is operating in the bcl-2-negative primitive hematopoietic precursors. Third, we will test different bcl-2- related genes for selective effects on hematopoietic differentiation using FDCP-Mix and EML cells. Finally, we will examine the bcl-2-related genes for functional differences in a bone marrow reconstitution model. Increased knowledge of how these bcl-2-related genes regulate cell survival and functionally interact with differentiation pathways may lead to novel approaches for maintaining hematopoietic stem cell viability and proliferation.